1. Field of the Invention
The present invention relates generally to a device and method for measuring pressures, and more particularly, to a method for performing more accurate measurement at low cost.
2. Background of the Invention
One type of conventional pressure measuring device receives an analog output produced by a semiconductor strain gauge and converts it to a digital output. This type of device has a resistor layer formed on a surface of a semiconductor such as silicon and measures a change in resistance caused by an induced strain on the semiconductor by an applied pressure. An output is obtained through a bridge circuit configured with a plurality of resistor layers for temperature compensation. The pressure measuring device utilizing a semiconductor strain gauge provides a measurement of high accuracy by means of the bridge circuit. A drawback of the device, however, is that it is very expensive to manufacture because of high cost of a strain gauge and AD converter.
Another type of pressure measuring device, which can be manufactured at a reduced cost, measures a frequency of a quartz resonator. More particularly, this type of device measures the difference in oscillation frequency between two quartz resonators: one placed in a reference pressure such as vacuum or air atmosphere and the other placed under an applied pressure to be measured. This type of devices are disclosed in, for example, Japanese laid-open patents SHO 54-158275, SHO 57-12342, SHO 59-67437, and HEI 3-248028. Although these devices can be manufactured at low cost, they do not necessarily provide an accurate pressure measurement on account of temperature characteristics of the quartz resonators. Particularly, if inexpensive quartz resonators are used, measurement wildly varies.
A method for improving temperature characteristics of pressure measuring devices utilizing quartz resonators is demonstrated in Japanese laid-open patent HEI 3-189528. In this method difference in oscillation frequency between two oscillation circuits, one having a quartz resonator placed in vacuum and the other having another quartz resonator in an atmospheric pressure, is counted by a counter. A pressure measuring unit computes the pressure using the measured count and also performs temperature calibration in use of a linear mathematical formula, which has been determined in advance from four count measurements under two known pressures and two known temperatures. The calibration formula is obtained from the four pairs of data on the assumption that the difference in frequency between the two oscillation circuits is linear with temperature and that the formula itself is a linear function of temperature.
However, the temperature characteristic varies depending on different intrinsic physical properties of individual quartz resonators and on different behaviors of quartz resonators under an air atmospheric pressure and vacuum. Therefore, even the aforementioned linear calibration formula does not provide accuracy of 1 hPa, which is needed for an atmospheric pressure measurement. Further, according to this method frequencies of a quartz resonator at different pressures and temperatures must be counted and calibration factors must be determined before shipment, resulting in increased cost. If one measures the characteristic of a typical quartz resonator at different pressures and temperatures only once, and applies the same calibration factors to every product, the temperature calibration will not allow for good accuracy because of variation of characteristics of individual quartz resonators assembled in pressure measuring devices.
What is needed then is a method for improving the accuracy of pressure measurement performed in use of quartz resonators by means of appropriate temperature calibration and for implementing the temperature calibration at lower cost.